1. Field of the Invention
This invention relates generally to electronic sensors and more specifically to optical position sensors.
2. Description of Related Art
For many years solid state position sensitive detectors have been used to monitor the location of an optical spot that is incident upon the active surface of a device (B. Light, Lasers and Applications 5 (4):75, April 1986). Two types of monolithic photodetectors are commercially available for measuring displacements in one dimension. The lateral effect detector incorporates an electrically resistive layer over the active surface area of a single photodiode, with electrical contacts at either end of the layer (J. T. Wallmark, Proc. IRE 45:474 1957). This type of detector is useful for measuring the centroid of an optical spot that may move across the entire photosensitive area. A second type, called the bi-cell, is sensitive to displacements that are small compared to the size of the optical spot, and commonly is used to monitor perturbations of a probe beam caused by mechanical vibration or optical misalignment. The circuitry of the bi-cell is shown in FIG. 1A. The bi-cell sensitivity is sufficient for use in atomic force microscopy, in which an optical beam is reflected off of the back surface of a small contact probe that is scanned across a solid sample surface D. Sarid, Scanning Force Microscopy: with Applications to Electric, Magnetic, and Atomic Forces: Revised Edition, (Oxford University Press, New York, 1994), Chap. 10, pp. 119-128!. By monitoring the deflection of the beam, it is possible to generate surface topography images with atomic resolution. Position sensitive detectors also are used for observing the "mirage effect," upon which photothermal deflection spectroscopy is based. This is described by W. B. Jackson, N. M. Amer, A. C. Boccara, and D. Fournier, in Appl. Opt. 20, 1333 (1981) and by R. E. Russo, F. R. McLarnon, J. D. Spear, and E. J. Cairns, in the J. Electrochem. Soc. 134, 2783 (1987). In photothermal deflection spectroscopy, a sample absorbs excitation radiation, producing thermal gradients in or adjacent to the sample. Refractive index gradients accompany the thermal gradients, causing the deflection of an optical probe beam, which is monitored as a measure of radiative absorption. The position sensitive detector reported in this work has been developed specifically for increasing the capability of optical probe beam deflection measurements.
The conventional bi-cell optical position sensor consists of two photodiode segments manufactured from a single piece of doped semiconductor material. Like the lateral detector, the bi-cell uses three electrical leads. One lead is common to both sides of the detector, and the other two provide separate paths for the photocurrent, allowing for discrimination based on position. Both designs require two op-amps for electronic amplification, with feedback resistors R, measured in ohms (.OMEGA.), for converting the currents produced by the photodiode segments into measurable voltages J. G. Graeme, EDN 32:229 Nov. 26, 1987; and J. G. Graeme, Photodiode Amplifiers: Op Amp Solutions, (McGraw-Hill, New York, 1996), Chap. 10, pp. 221-243!. FIG. 1A shows the basic circuit for these commercially available sensors. The bi-cell photodiodes 2 and 4 are made as an integrated circuit on a single silicon wafer and thus are immediately adjacent to one another. As a beam moves across the two photodiodes, the amount of light falling on each photodiode 2 and 4 is amplified by an op-amp 6 and 8 and compared. The difference between the op-amp output voltages, (V.sub.2 -V.sub.1), is taken as a measure of the deflection of the position of the incident optical beam. It is possible also to use the sum of the two op-amp output voltages, (V.sub.1 +V.sub.2), as a measurement of optical power, for normalization purposes. Under optimal design conditions, the dominant source of electrical output noise, Vn, measured in volts (V), for each op-amp in the circuit is the thermal noise, also sometimes referred to as the Johnson noise, which originates from the feedback resistors 10 and 12 Noise analysis of FET transimpedance amplifiers, in The Handbook of Linear IC Applications, pp. 187-190, Burr-Brown Corp., Tucson (1987)!: ##EQU1## where k is Boltzmann's constant (1.38.times.10.sup.-23 J/K), T is the temperature in degrees Kelvin (K), B is the noise bandwidth in Hertz (Hz), and R is the feedback resistance (.OMEGA.). In this type of current-to-voltage amplifier, the value of the feedback resistor R is equal to the gain of the circuit in units of volts per ampere. Therefore, the effective current measurement noise, i.sub.n, measured in amps (A), is inversely proportional to the square root of the gain: ##EQU2##
Equation (2) indicates that maximizing the value of the feedback resistors minimizes electrical noise in a measurement of photocurrents. However, a practical limitation on increasing the value of R is that the output of each op-amp can become saturated. For example, if a conventional position sensitive detector is used to monitor the position of a 2 milliwatt (mW) helium-neon laser beam (.lambda.=633 nm), and the sensitivity of the detector at this wavelength is 0.33 A/W (typical for silicon photodiodes), then the photocurrent input to each op-amp is equal to 0.33 mA. An op-amp might specify a maximum output voltage of 10 V, which would limit the gain of each op-amp circuit to no more than 3.times.10.sup.4 V/A. The amount that the electrical noise can be reduced, simply by increasing R, is thus limited.
Hamamatsu describes using its photodiodes in a light balance detection circuit so that intensity of light having different wavelengths can be balanced (Catalog No. KPD0001E03, page 51). The Hamamatsu circuit does not detect the position of an optical beam and none of the optical design specifications necessary for using the circuit to detect beam position are provided.
Jerry G. Graeme Photodiode Amplifiers: Op Amp Solutions, (McGraw-Hill, New York, 1996), Chap. 10, pp. 221-243! describes a variety of circuits using photodiodes for optical position sensing. His discussion however omits mention of specific designs or problems with noise associated with use of two op amps. For high sensitivity measurements of an optical beam, conventional bicells having the standard two op-amp circuitry are considered to be the best measuring device.
Optical position sensors have in the past been constructed using an electrical component configuration that requires two current amplifiers and one differential voltage amplifier. This configuration introduces unwanted electrical noise into the system. It would be highly desirable to be able to measure the position of an optical spot with less electrical noise than has been possible with currently available circuitry.